1. Field of the Invention
The present invention relates to an image forming apparatus applicable to printing units of digital copying machines and facsimile machines, as well as to digital printers, plotters, or the like, and forming an image onto a recording medium by scattering a developer.
2. Description of the Related Art
There are known as image forming methods for forming visible images onto a recording medium such as paper by using electric signals outputted from a computer, a word processor, a facsimile machine, etc., an ink jet method using ink, a heat transfer method of transferring an image with fused ink, an ink sublimation method, an electrophotographic method etc.
In recent years, in order to meet demands for faster printing, higher printing quality and lower prices of printing, the ink jet method, which is one of non-impact image forming methods that allows the apparatus to be manufactured integrally with a print head in a comparatively simplified configuration, is increasingly adopted. However, since a liquid of ink is used for the ink jet method, the ink is apt to ooze out on paper and a satisfactory image is difficult to obtain. Besides, in the case of color overprinting, mixed colors made by mixing inks cannot be obtained as expected. Therefore, the electrophotographic method with toner is adopted when high quality printing is required. In printing with toner, the toner does not ooze out, and hence an image which has a thick color tone and is excellent in visual appeal can be obtained. In addition, in the case where a plurality of colors are mixed for the purpose of color printing, the colors are mixed in the fixing process and satisfactory mixed colors can be obtained. Accordingly, there is proposed a direct printing method with toner, which is a combination of simple processes of the ink jet printing method and toner images.
For example, Japanese Unexamined Patent Publication JP-A 4-358856 (1992) discloses a method for forming an image in an image forming apparatus comprising an aperture electrode unit having openings, a toner holder for feeding charged toner to the vicinity of the openings, and a counter electrode facing the aperture electrode unit, which is positioned at the opposite side of the toner holder, the method comprising modulating and controlling the toner held on the toner holder by the aperture electrode unit.
In the technology disclosed in JP-A 4-358856, the image forming apparatus is formed in such a manner that an oscillating electrical field is generated at a piezoelectric member provided so as to cover the toner holder to separate the toner from the toner holder due to the expansion/shrinkage of the piezoelectric member according to the oscillating electrical field. In this case, since the whole toner holder is vibrated, even the toner held on the toner holder in areas which are neither to be modulated nor to be controlled by the aperture electrode unit is separated from the toner holder, with the result that the toner is likely to be accumulated on the aperture electrode unit.
Further, since it is difficult to finely control the toner by vibration, it is very difficult to keep the toner between the toner holder and the aperture electrode unit without causing the toner to fall onto the aperture electrode unit. Furthermore, means for generating an oscillating electrical field at the piezoelectric member is formed to be an oscillating power supply for generating the oscillating electrical field, but it makes the circuit configuration complicated and leads the manufacturing cost of the apparatus to increase. Therefore, such a method is not favorable.